The motion picture industry is presently transitioning from traditional film based projectors to digital or electronic cinema. This trend is accelerating due to the popularity of 3-D movies. Even as digital cinema projection has matured and succeeded, largely based on the use of the well-known Digital Light Projection (DLP) technology, the promise of a further evolution to laser-based projection has been hovering in the background. Laser projection, whether for digital cinema, home projection, or other markets, has long been held back due to the cost and complexity of the laser sources, particularly in the green and blue spectral bands. As the necessary lasers are now becoming increasingly mature and cost competitive, the potential benefits expected from laser projection, including the larger color gamut, more vivid, saturated and brighter colors, high contrast, and low cost optics are increasingly being realized. An exemplary system is described in the paper “A Laser-Based Digital Cinema Projector”, by B. Silverstein et al. (SID Symposium Digest, Vol. 42, pp. 326-329, 2011).
Most commonly in cinema, stereo projection has been enabled by polarization techniques, where image content to the left and right eyes is projected using orthogonal polarization states (e.g., horizontal linear and vertical linear polarizations), and viewers wear corresponding glasses. Light of one polarization is transmitted, and light of the opposite polarization is blocked, and the crosstalk between left and right eye images is ideally ≧150:1 for all fields of view. For example, U.S. Pat. No. 4,957,361 (W. Shaw) to IMAX Corp. of Mississauga ONT, CA, provided spectacles with left and right eye filters that are polarized at right angles to each other, and which produce the perception of depth when viewing motion pictures with double images that are likewise polarized at right angles to each other. The laser projector of Silverstein et al., provided linear polarized image light that that worked with such stereo glasses. Alternately, RealD Inc. of Boulder Colo. has commercialized post-projector polarization, using for example the Z-Screen modulator and circularly polarized glasses, the latter described in U.S. Pat. No. 7,524,053 (L. Lipton).
The earliest form of stereo was the anaglyph, first developed by L. du Hauron in 1894. In the traditional printed anaglyph, each eye only sees a color adjacent subset of the visual spectrum (e.g., red & cyan), as defined by broad spectrum dye based color filters, although the viewer perceives a black and white or tinted image. As exemplified by the Color Code system of U.S. Pat. No. 6,687,003 (S. Sorenson et al.), anaglyph glasses have been developed with alternate broad band color filter pairs (e.g., amber and dark blue) that are specified by transmission characteristics to provide both 3D and improved color perception. Likewise, the INFICOLOR approach of US 20100289877 (Lanfranchi et al.) uses a green and magenta color filter pair, using broad band filters from Lee Filters (Andover, UK) or Rosco Laboratories (Stamford, Conn.) to provide anaglyphs with improved color perception.
More recently, the traditional stereo image approach of anaglyph color coding has been extended to electronic displays and cinema. The most common such approach is spectral separation or wavelength multiplex visualization, where the display provides spectral coded output as spectral triplets, R1G1B1 and R2G2B2, and the viewer wears glasses where one eye sees one spectral triplet (R1G1B1) and the other eye sees the second spectral triplet (R2G2B2). This wavelength triplet approach provides an improved sense of color perception, as each eye sees all three colors. Also, wavelength triplet images can be more acceptably viewed as 2D images, as this spectral color coding is subtler than the anaglyphic spatial color coding approach. As one example, U.S. Pat. No. 6,698,890 (H. Jorke) to Daimler Chrysler, provides a color sequential projector (FIG. 4 thereof) having a lamp source and filters, which creates 6 primaries in two sets, alternating R1 G1 B1 and R2 G2 B2 spectra, each primary being 20 nm wide, for stereoscopic viewing using glasses constructed with interference filters. Jorke '890 provides exemplary spectral bandwidths Δλ that are 435-455 nm, 510-530 nm, 600-620 nm, and 460-480 nm, 535-555 nm, 625-645 nm, respectively.
This spectral multiplexing approach, which is generally known as “6P” for use of six primaries, has been further developed. For example, it has been observed that the spectral filters for the projector and glasses of Jorke '890 have steep spectral edges and are hard to fabricate; and particularly in the case of the glasses, the coatings can cause color image artifacts (such as crosstalk) and hue differences. As an improvement, U.S. Pat. No. 7,832,869 (Maximus et al.) provides a stereoscopic projector where color switching enables rapid switching or cycling between left eye and right eye image projection. As another improvement, U.S. Pat. No. 7,784,938, (Richards et al.) to Dolby Laboratories, provides 6P stereo glasses having dichroic interference filters, where the projector filters are pre-blue shifted and the glasses filters have coatings that are formed on curved lenses with improved guard bands and variable thickness coatings, to reduce 3D crosstalk of image content from the target eye to the other eye.
The problem of spectral color shift with angle is inherent to dichroic interference filters. For example, the paper, “Tunable thin-film filters: review and perspectives”, by Michel Lequime, SPIE Proc. 5250, pp. 302-311, 2004, provides an equation describing the spectral shift:λθ=λ0(1−sin(θ)2/neff2)1/2 
In this equation, λ0 is the center wavelength of the filter at normal incidence, λθ is the center wavelength of the filter at oblique incidence, θ the angle of incidence of the collimated light beam in air, and neff is the effective index of refraction of the filter. This last quantity depends of the nature of the spacers (high- or low-index) of the elementary Fabry-Perot cavities used in the design of our narrowband thin-film filter and varies with m, the interference order of these cavities and nH and nL, the refractive indices of the quarter-wave alternated layers used for the realization of their high reflectance mirrors. In general, to reduce the wavelength shift for a given angle of incidence in air, the effective index needs to be increased by preferentially using high-index materials and a low interference order. Nonetheless, for angles of incidence above ˜30°, coating edges can spectrally shift (Δλs=λθ−λ0) by 15 nm or more, as illustrated in the spectral graph of FIG. 7A, where a dichroic pass band 393 shifts to shorter wavelengths with increasing angle, becoming shifted dichroic pass bands 394. Such spectral shifts can cause crosstalk problems (<50:1 contrast) if the two spectral channels are separated by a small spectral gap 135, as are the green spectral pair (G1 and G2), which are only 15-20 nm apart. Moreover, as the FOV increases, transmission typically also drops off, from ˜80-90% on axis to <30-50% off axis. These transmission and crosstalk variations can cause problem in theatres, and particularly in large screen theaters that support fields of view (FOV) of ±40° or more.
Given these angular problems, and the fact that dichroic glasses have coatings that are formed by thin film deposition in vacuum chambers, it would be desirable to provide 6P glasses by other means. Notably, US Pat. Pub. 20120307358 (M. Baum et al.), suggests that angularly independent color filter bands can be generated by absorption color filters, in a manner similar to anaglyphs. However, the color filters of Baum '358 are portrayed as “cliff functions” with unrealistically straight edges—as evidenced by comparison to the complex filter spectra provided in the previously discussed U.S. Pat. No. 6,687,003 (S. Sorenson et al.) and US 20100289877 (Lanfranchi et al.). Additionally, Baum '358 is vague on how to produce these filters, citing the manner of film production as being sufficient. In conclusion, there is a need for alternative 6P dichroic glasses, preferably being both less costly and having improved performance at larger angles of incidence, than is available from 6P dichroic glasses, or than has been provided thus far by absorptive glasses.